Plural electrode development methods for latent electrostatic images

ABSTRACT

Development methods and develoment electrode apparatus therefor are provided in accordance with the teachings of the present invention wherein at least two development electrodes are employed at a developing station in electrophotographic apparatus. The first development electrode presented to a latent electrostatic image undergoing development displays characteristics which are optimum for high rates of developer flow and hence only reduced development of portions of a latent electrostatic image exhibiting low charge densities and extended areas is achieved but may be accompanied by optimum cleaning. The second development electrode presented to the instant electrostatic image to be developed displays characteristics which are optimized to complete low contrast and extended area development, however, as the majority of developer material is removed, no inhibition of the developer flow occurs at said second development electrode.

United States Patent 1191 Cade et al.

1 11 3,807,997 1 Apr. 30, 1974 PLURAL ELECTRODE DEVELOPMENT METHODS FORLATENT ELECTROSTATIC IMAGES [75] Inventors: Ronald L. Cade, Fairport;John F.

Knapp, Webster, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: Dec. 22, 1972 1 [21] Appl. No.: 317,573

Related US. Application Data [62] Division-of Ser. No. 141,240, May 7,1971, Pat. No.

Primary ExaminerRonald H. Smith Assistant Examiner-Edward C. KimlinABSTRACT Development methods and develoment electrode apparatus thereforare provided in accordance with the teachings of the present inventionwherein at least two development electrodes are employed at a developingstation in electrophotographic apparatus. The first developmentelectrode presented to a latent electrostatic image undergoingdevelopment displays characteristics which are optimum for high rates ofdeveloper flow and hence only reduced development of portions of alatent electrostatic image exhibiting low charge densities and extendedareas is achieved but may be accompanied by optimum cleaning. The seconddevelopment electrode presented to the instant electrostatic image to bedeveloped displays characteristics which are optimized to complete lowcontrast and extended area development, however, as the majority ofdeveloper material is removed, no inhibition of the developer flowoccurs at said second development electrode.

4 Claims, 2 Drawing Figures Cleaning Station mgm menao I974 3807l997 sum1 or 2 Cleaning S'rotion PATENTEDAPRBO m4 3807' 997 SHEET 2 OF 2 Fig. 2.

PLURAL ELECTRODE DEVELOPMENT METHODS FOR LATENT ELECTROSTATIC IMAGESThis is a division, of application Ser. No. 141,240, filed May 7, 1971,now U.S. Pat. No. 3,719,169.

This invention relates to electrophotographic development techniques andmore particularly to improved methods and apparatus for developinglatent electrostatic images.

In conventional electrophotographic processes such as taught in US. Pat.No. 2,297,691, as issued to Carlson on Oct. 6, 1947, or as contemplatedby other electrophotographic techniques well known to those of ordinaryskill in the art, a photoreceptor is charged and selectively exposed sothat a latent electrostatic image representative of the object to becopied is formed due to the selective discharge of the surface of thephotoreceptor in accordance with a light and dark pattern representativeof the object to be copied. The latent electrostatic image formed isthen developed or rendered viewable generally by the application ofcharged finely divided particles, known as toner, to the image area ofthe photoreceptor so that such charged particles are deposited in aselective pattern on the image area of the photoreceptor in accordancewith the charge pattern exhibited by the latent electrostatic imageformed. The developed image as thus fonned may then be fixed to thesurface of the photoreceptor or as is usually the case transferred to atransfer member, such as a paper sheet, and then fixed so that thephotoreceptor is available for subsequent use.

In the development of a latent electrostatic image by the application ofcharged toner particles to the surface of a photoreceptor, the contourof the electric field in the region of the latent electrostatic imageplays a critical role in the manner in which toner particles will beattracted to' and deposited on the surface of the photoreceptor. Moreparticularly, in areas where the latent electrostatic image exhibitssharp electrostatic contrast, such as a portion thereof corresponding toa boundary between a light and dark portion of the object being copied,and is thus characterized by a fringing field, toner particles willdensely adhere. However, in areas corresponding to extended dark areasof the object being copied, afringing field configuration will onlyobtain at the edges thereof and hence when such an area is developed bythe application of oppositely charged toner particles thereto, the edgesof such an area will develop out as solid black due to a high densitydeposition of charged toner particles thereon while portions of thephotoreceptor central to such an area would not receive a high densitytoner deposition and accordingly would not develop out as solid black.This development phenomenon obtains because of the different chargelevels, and hence the greater field intensity or sharp voltage contrastexhibited at the boundaries between light and dark portions of anelectrostatic image and due to the fact that more toner particles willbe deposited in regions of sharp potential difference. Therefore, in thecentral portions of a large solid image area, the voltage contrastbetween adjacent points is small and toner deposition during developmentis proportionally reduced.

While the foregoing development phenomenon is advantageous with respectto the development of latent electrostatic images representing line copyor the like due to the high contrast development which results, the samephenomenon would virtually preclude electrophotographic copying ofobjects manifesting continuous tone qualities and particularly thosecharacterized by large solid areas associated with a dark image patternbecause the resulting developed image derived therefrom would be markedby a halo effect and generally convey a washed-out appearance. However,as is well known to those of ordinary skill in the art, the adverseeffects characterizing the foregoing development phenomenon may becompletely alleviated by using a device known as a developmentelectrode. Development electrodes in their simplest configuration maytake the form of a conductive sheet disposed in close proximity to andparallel with the surface of a photoreceptor on which an electrostaticcharge pattern is formed. The development electrode is normally biasedwith respect to the charge pattern or latent electrostatic image formedon the photoreceptor and such bias may conveniently be selected to beequal to the background level of the latent electrostatic image so thatthe background will develop out at a uniform white level. The effect ofthe development electrode is to change the field configurationassociated with the latent elecrostatic image and to effectively modifythe field in the space above large continuous areas of charge. Thus,development electrodes act to vary the field configuration associatedwith the latent electrostatic image so that the various portions of suchlatent electrostatic image will be developed in a manner more nearly inproportion to the electrical charge density thereof rather than in arelationship proportional to the voltage gradients which obtain.Additionally, a development electrode acts to intensify the electricalfield near the surface of the photoreceptor so that field strength atany point between the surface of the photoreceptor and the developmentelectrode will be proportional to the potential exhibited by the surfaceof the photoreceptor on which the latent electrostatic image is formedand inversely proportional to the distance between the photoreceptor andthe development electrode. Thus, for effective utilization, adevelopment electrode must be disposed in a relatively closerelationship to the photoreceptor on which the latent electrostaticimage is fonned. Therefore, as is well known to those of ordinary skillin the art, the use of a development electrode or a device incorporatingthe attributes thereof, is virtually mandatory to high qualitycontinuous tone development of latent electrostatic images and to thereproduction of solid black areas of an image to be developed.

Of the many development techniques well known to those of ordinary skillin the art, modified cascade development techniques have found wideareas of application and are in general use in electrophotographicprocessing equipments due principally to the ease with which largevolumes of toner materials may be applied to the surface of aphotoreceptor whereby potentially high density image development may beachieved. Cascade development techniques are based upontriboelectrification principles whereby two dissimilar materials arebrought into contact with each other and each material exhibits aresulting charge polarity opposite to the other. Accordingly, in cascadedevelopment techniques, relatively fine toner particles are mixed with acoarse, beaded material called carrier, as taught for instance in US.Patl No. 2,297,691, to form developer material whereupon the tonerparticles become triboelectrically charged and adhere to the surfaces ofthe carrier beads. Development of a latent electrostatic image may thenbe carried out by cascading the developer material formed over thesurface of the photoreceptor and as the carrier beads pass over theportions of the surface of the photoreceptor occupied by the la tentelectrostatic image, the electrostatic forces manifested by the latentelectrostatic image will overcome the electrostatic bond between thetoner particles and carrier beads so that such toner particles will bedeposited on the image area of the photoreceptor. The cascading of thedeveloper material may be accomplished by simply pouring developermaterial over a photoreceptor in the form of a plate and rocking thephotoreceptor to accomplish developer motion and carrier bead removal.Alternatively, in continuous process electrophotographic copyingapparatus wherein the photoreceptor normally takes the form of a drum ora continuous web, the developer material is flowed over the surface ofthe photoreceptor using a gravitational flow and the large amount oftoner and bead material recovered therefrom is recycled for subsequentuse.

The cascade development technique briefly outlined above presents arather ideal mode of development for latent electrostatic imagesrepresenting line copy and the like because the field configurationassociated therewith, as aforesaid, coupled with the large amount ofavailable toner particles applied thereto results in a viewable imageconfiguration having solidly black characteristics and sharp contrast.However, when cascade development techniques are used in conjunctionwith latent electrostatic images exhibiting continuous tonecharacteristics and/or having large or solid black areas associatedtherewith, the resulting image developed will manifest the washed-outand haloed appearance described above. Accordingly, for the successfulapplication of cascade development techniques to the development oflatent electrostatic images exhibiting continuous tone, development musttake place in the presence of a development electrode for the reasonsdetailed above.

In a typical two component developer materials in general use, thecarrier beads employed to triboelectrically charge the toner materialdisplay a typical size variation ranging from 250 to 600 ,um in diameteral' though carrier beads as small as 100 um are known. Therefore, whendeveloper employing carrier material of this nature is utilized incontinuous process electrophotographic equipment, it will be appreciatedthat if a development electrode is placed sufficiently close to thesurface of the photoreceptor so that field varying characteristicsthereof have a substantial effect, the developer material will have atendency to bunch between the surface of the photoreceptor and thedevelopment electrode. This bunching of the developer material not onlyinhibits the free fiow of developer necessary for proper development incascade development applications, but in addition thereto often resultsin smudging of the developed image, scratching and deterioration due toabrasion of the surface of the photoreceptor together with significantlyreduced development times. As cascade development techniques aregenerally preferred, substantial efforts have been devoted to solvingthe problem posed by the conflicting requirements of a location for thedevelopment electrode proximate to the surface of the photoreceptor andthe use of appropriate volumes of developer material which are freelycascaded or flowed over the surface of the photoreceptor so that adensely populated, rapidly developed toner image may be formed. However,as the spatial requirements of a properly positioned developmentelectrode, as aforesaid, are generally adverse to the requirements ofcascade development techniques, the solutions presently available havetended to proceed by way of compromising the parameters employed forboth the development electrode relied upon and the cascade developmenttechnique employed. Thus, in certain electrophotographic copyingsystems, smaller volumes of developer material are cascaded over thesurface of the photoreceptor in an effort to reduce the bunching effectcaused by the development electrode and the development electrode isplaced a further distance away from the surface of the photoreceptorthan would otherwise be selected; however, this compromise has not beenhighly desirable because it results in development speeds and hencemachine speeds which are greatly reduced while the effects produced bythe development electrode are attenuated. Other solutions proceed byusing specialized development electrodes, as disclosed for instance inU.S. Pat. Nos. 3,147,147, to C. F. Carlson, and 3,011,474, to H. O.Ulrich wherein the development electrode is apertured or laminated inform and thus configurated in a manner to optimize the flow of developermaterial while development electrode spacings are employed which arecalculated to avoid bunching of the developer material. However, asgrooved, laminated or apertured development electrodes are expensive tomanufacture and must be rather large in surface area due to thediscontinuous surfaces employed, and furthermore since such specializeddevelopment electrodes may not be closely disposed to the surface of thephotoreceptor on which the latent electrostatic image is formed, thesesolutions have been viewed as less than ideal.

Additionally, although virtually all continuous processelectrophotographic equipments in general use employ a separate cleaningstation to remove residual toner particles from the surface of areusable photoreceptor prior to each use thereof, recent advances in theart of cascade development techniques, as set forth in Application Ser.No. 789,031 to R. L. Cade and S. W. Volkers entitled lmaging Systems asfiled on Dec. 31, 1968 and assigned to the Xerox Corporation, haveindicated that the step of cleaning or the removal of residual tonerparticles from the surface of the photoreceptor may be accomplishedsubsequent to the formation of a new latent electrostatic image andduring the development of such new latent electrostatic image. Thistechnique of development cleaning is considered to possess potentialattributes which would be highly desirable in continuous processelectrophotographic equipment because successful development cleaningtechniques would enable the deletion of specialized cleaning apparatusfrom continuous process electrophotographic equipment employing cascadedevelopment techniques. Thus, not only would the resulting structure ofelectrophotographic equipments employing development cleaning be highlysimplified, but in addition thereto, the manufacturing and maintenancecosts associated with a separate cleaning stage would be avoided.

Therefore, it is an object of this invention to provide developmentmethods and development electrode apparatus therefor for use indeveloping latent electrostatic images wherein field variations affectedby the development electrode configuration employed aremaximized whilelarge volumes of toner materials may be applied during the developmentof such latent electrostatic image.

A further object of this invention is to provide development cleaningmethods and apparatus therefor wherein residual toner particles from apreceding development may be removed while high quality, continuous tonedevelopment of a latent electrostatic image is obtained.

A further object of this invention is to provide split electrodedeveloping apparatus wherein the characteristics of one portion of saidsplit electrode is optimized for use with large volumes of developermaterial having a high flow rate while another portion of said splitelectrode has characteristics which are optimum for the development oflow density and extended area portions of a latent electrostatic imagebut may only receive developer material at relatively reduced flowrates.

Various other objects and advantages of the present invention willbecome clear from the following detailed description of severalexemplary embodiments thereof, and the novel features will beparticularly pointed out in conjunction with the claims appended hereto.

In accordance with the teachings of the present invention developmentmethods and development electrode apparatus therefor are providedwherein at least two development electrodes are employed at a developingstation of electrophotographic apparatus, the first developmentelectrode presented to a latent electrostatic image to be developeddisplays characteristics which are optimum for high rates of developerflow and hence only reduced development of low charge densities andextended image areas is achieved but may be accompanied by optimumcleaning; the second development electrode presented to said latentelectrostatic image to be developed displays characteristics which areoptimized to complete low contrast and extended area development,however, as the majority of developer material is already removed, nobunching or inhibition of the developer flow occurs at said seconddevelopment electrode. The invention will be more clearly understood byreference to the following detailed description of several exemplaryembodiments thereof in conjunction with the accompanying drawing inwhich:

FIG. 1 illustrates an exemplary embodiment of this invention wherein themethods of development and apparatus therefor as taught herein are shownin combination with typical electrophotographic equipment; and

FIG. 2 illustrates another exemplary embodiment of this inventionwherein methods of development cleaning and apparatus therefor ascontemplated herein are shown in combination with a modified version ofthe electrophotographic equipment shown in FIG. 1.

Referring now to the drawings and more particularly to FIG. 1 thereof,there is shown an exemplary embodiment ofthe present invention whereinthe methods of development contemplated herein are embodied in exemplary development electrode structure therefor and depicted inconjunction with typical electrophotographic processing equipment. Theelectrophotographic processing equipment illustrated in FIG. 1 anddescribed below has been set forth because the methods of developmentand the development electrode structure therefor taught by thisinvention are considered to best admit of a full and complete disclosurewithin an environment in which they might ordinarily be expected tofunction; however, as will be readily appreciated by those of ordinaryskill in the art, the details of the electrophotography processingequipment, and the processes set forth form no part of the presentinvention per se and accordingly are disclosed for the purposes ofexplanation rather than limitation.

As shown in FIG. I, the electrophotographic equipment illustrated takesthe form of continuous processing electrophotographic apparatus based onthe concepts originally disclosed in US. Pat. No. 2,297,691, issued toCarlson on Oct. 6, I942, and accordingly comprises a photoreceptor 2, acharging station 4, an exposure station 6, a development station 8, atransfer station 10 and a cleaning station 12. The photoreceptor 2 asillustrated in FIG. 1 may take the conventional form of a drum orendless web adapted to rotate in the direction indicated by the arrow Aor alternatively, the photoreceptor 2 may take the form of a web, sheetor plate arranged to be conveyed past horizontally disposed processingstations by conventional winding, reeling or conveying techniques. Inany event, the photoreceptor 2 may take any well-known structuralconfiguration and for the purpose of the instant disclosure has beenillustrated as a simple two layer drum 2 including an insulating member14 and a conductive member 16. As the photoreceptor 2 forms no part ofthe instant invention per se, it is here sufficient for an appropriateunderstanding of this disclosure to appreciate that if the embodiment ofthis invention depicted in FIG. 1 is employed in conjunction withconventional electrophotographic techniques, the conductive memer 16 maybe formed of any suitable conductive material or a nonconductorovercoated with a conductive foil. Similarly, the insulating member 14would ordinarily be formed of a material displaying photoconductivecharacteristics such that the member 14 is normally insulating andexhibits excellent charge retentivity but may be rendered selectivelyconductive by the application of electromagnetic radiation theretothrough a light and dark pattern representing an object to be copied oralternatively, reflection exposure techniques may be employed. Thematerials relied upon in the formation of the insulating member 14 maybe selected from any of the well-known group of materials conventionallyemployed in electrophotographic processes such as amorphous selenium,alloys of sulfur, arsenic or tellurium with selenium doped withmaterials such as thallium, cadmium sulfide, cadmium selenide, etc.,particulate photoconductive materials such as zinc sulfide, zinc cadmiumsulfide, French process zinc oxide, phthalocyanine, cadmium sulfide,cadmium selenide, zinc silicate, cadmium sulfoselenide, linearquinacridones, etc., dispersedin an insulating inorganic film formingbinder such as a glass or an insulating organic film forming binder suchas an epoxy resin, a silicone resin, an alkyd resin, a styrenebutadieneresin, a wax or the like. Other typical photoconductive insulatingmaterials include: blends, copolymers, terpolymers, etc., ofphotoconductors and non-photoconductive materials which are eithercopolymerizable or miscible together to form solid solutions and organicphotoconductive materials of this type include: anthracene,polyvinylanthracene, anthraquinone, oxidiazole derivatives such as2,5-bis-(p-aminophenyl)-l ,3,4- oxadiazole; 2-phenylbenzoxazole; andcharge transfer complexes made by complexing resins such as polyvinylcarbazole, phenolaldehydes, epoxies, phenoxies, polycarbonates, etc.,with Lewis acids such as phthalic anhydride; 2,4,7-trinitroflourenone;metallic chlorides such as aluminum, zinc or ferric chloride; 4,4-bis(dimethylamino) benzophenone; chloranil, picric acid;1,3,5-trinitrobenzene; l-chloranthraquinone; bromal;4-nitrobenzaldehyde; 4-nitrophenol; acetic anhydride; maleic anhydride;boron trichlroide; maleic acid, cinnamic acid; benzoinc acid; tartaricacid; malonic acid and mixtures thereof. The insulating member 14 may bemade either transparent or radiation absorbing in nature by the choiceof he photoconductive insulating material selected and as will beappreciated by those of ordinary skill in the art; a multi-layerinsulating material could be readily substituted for the single layermember 14 illustrated in FIG. 1. The photoreceptor 2 is disposed in anoperative relationship with each of the processing stations arrangedabout the periphery thereof so that upon appropriate energization of theillustrated electrophotographic equipment, the rotation of thephotoreceptor 2 in the direction indicated by the arrow A, as aforesaid,will subject each point on the periphery of the insulating member 14 tothe process step performed at each such station.

The charging station 4 may take the conventional form of one or morecharging devices 18-20 which act in the well-known manner to sensitizethe photoreceptor 2 by charging the surface of the insulating member 14to a uniform potential. Although any conventional form of chargingdevices l820 may here be relied upon to impose a charge level on thesurface of the photoreceptor 2, corotron devices having half-roundshield configurations have been illustrated in FIG. I because theutilization of corotrons in typical electrophotographic equipment isusually preferred. The structure and mode of operation of corotrons suchas are illustrated in FIG. 1 are well known and are described in detailin US. Pat. Nos. 2,836,725, and 2,879,395, to Vyverberg and Walkup,respectively. Therefore, for the purposes of the instant disclosure, itis sufficient to appreciate that each of the coronodes 21-23 of thedepicted corotrons is commonly connected to an appropriate source ofhigh potential V and such corotrons act in the conventional manner toimpose a uniform potential charge on the surface of the photoreceptor 2disposed thereunder. In FIG. l, a plurality of charging devices 18-20has been illustrated since a multiple corotron configuration normallyprovides more uniform charging across the width of the photoreceptor 2than would be otherwise available. This occurs because ion chargingcurrent from the various portions of an individual coronode disposedlongitudinally across the width of a photoreceptor will not ordinarilybe uniform and thus the use of plural corotrons will tend to average thecharge delivered to various points across the width of the photoreceptorto a uniform level. However, if specialized charging apparatus is reliedupon or if minor nonuniformities in the level of charge imposed areacceptable, only a single corotron or other charging device may beemployed. Further, for the purposes of the instant disclosure, it may beassumed that the charging station 4 acts to charge the surface portionsof the photoreceptor 2 passing thereunder to a uniform level of onethousand volts (1,000V), although, as well known to those of ordinaryskill in the art, other arbitrary voltage levels may be selected.

The exposure station 6 is also disposed about the periphery of thephotoreceptor 2 and may take the conventional form of a projectionsystem or the like wherein optical trasmission or reflection techniquesare relied upon to image a light and dark pattern representing theobject to be copied onto the surface of the insulating memer 14 disposedthereunder. In FIG. 1, a slit exposure device has been generallyindicated, however, any optical system which relies upon lenses or thelike may be readily employed. Additionally, although the exposurestation 6 illustrated in FIG. ll, has been shown positioned in suchmanner that the rotating photoreceptor is initially sensitized by thecharging station 4 and subsequently exposed, it will be apparent thatthe processing steps of charging and exposure may be carried outsimultaneously by altering the position of the exposure station 6 and/orthe charging station 4. Fur thermore, although only the simplifiedprocessing steps of a single charging operation and a single exposureare depicted in FIG. 1, it will be obvious that additionalelectrophotographic processing steps such as another charging operationmay be employed in the formation of the latent electrostatic image orthe latent electrostatic image formed may be reversed or otherwise altered by the use of such additional processing steps as is well known tothose of ordinary skill in the art. The photoreceptor 2 is nextpresented to the developing station indicated generally at 8 where alatent electrostatic image formed is developed in accordance with theteachings of the present invention.

The development station 8, as illustrated in FIG. 1 comprises hoppermeans 24l suitable for subjecting appropriate portions of thephotoreceptor 2 to a cascade development step or the like, a split orfirst and second development electrode means 26 and 28 disposed in apredetermined relationship with the periphery of the insulating member14 so as to function in accordance with the development methods taughtherein and a reservoir 30 for supplying developer material. The hoppermeans 24 may take any of the conventional forms of devices generallyemployed in electrophotographic equipment to flow developer materialsonto selected portions of the photoreceptor 2 surface disposedthereunder. The hopper means 24 is filled to an appropriate level withconventional developer materials, indicated at 32, which include thepreviously described carrier beads and toner material. The hopper means24 also includes a ramp 34 which has a selected inclination so thatdeveloper materials from the hopper 24 will be flowed over the surfaceof the photoreceptor 2 then associated therewith at preferably high flowrate for the developer materials selected so that cascade develop mentmay be rapidly accomplished in a manner to yield high contrast images.The flow rate of developer mate rial from the hopper means 24 maytypically reside at a level of fifty grams per inch of photoreceptorlength per second (50 gms./in./ see.) if conventional glass core carrierbeads are employed in the developer material utilized whilesubstantially larger flow rates as defined by the foregoing units areemployed when conventional steel core carrier beads are relied upon.Thus, the rates at which developer material is cascaded over the sur'face of the photoreceptor 2 by the hopper means 24 are preferably highbut may vary between one and one hundred grams per inch per second(1-100 gms./in./- sec.) depending upon the developer material selected.Furthermore, although relatively high flow rates for the developermaterials are preferred, if lower speeds of rotation for thephotoreceptor 2 are acceptable, the flow of material from the hoppermeans 24 may be significantly reduced.

The reservoir 30 may be conventional in form and is disposed beneath theportion of the photoreceptor 2 which receives developer material fromthe hopper means 24. The reservoir 30 is ordinarily maintained filled toa predetermined level with developer material indicated at 32 and actsto receive residue developer material from the surface of thephotoreceptor 2 as well as carrier beads from which toner material hasbeen removed during the development process. Additionally, the reservoir30 is provided with means, not shown, through which toner material maybe periodically introduced so that the toner material expended duringdevelopment may be replaced and conventional conveyor means, not shown,is provided between the hopper means 24 and the reservoir 30 so thatdeveloper material 32 from the reservoir 30 is supplied to the hoppermeans 24 during such times as the' depicted electrophotographicequipment is energized to thereby maintain the level of the developermaterial 32 in the hopper means 24 at a predetermined level. A partition36 is additionally provided in the manner illustrated in FIG. 1 so as toextend from a lower portion of the ramp 34 of the hopper means 24 towardthe reservoir 30 so that a chamber isolating the photoreceptor 2 and thefirst and second development electrodes 26 and 28 from the conveyormeans, not shown, and developer material therein is established.Although, in FIG. .1, the hopper means 24 is relied upon to flowdeveloper material over the surface of the photoreceptor 2, it will beappreciated by those of ordinary skill in the art that the conveyormeans taken separately or in combination with a ramp could be reliedupon to cascade developer material 32 over the surface of photoreceptor2 as could any other conventional means well known to those of ordinaryskill in the art.

The first and second development electrodes 26 and 28 are disposedwithin the chamber formed by the partition 36 and are positionedopposite to peripheral surface portions of the photoreceptor 2 in themanner illustrated in FIG. 1. Each of the first and second developmentelectrodes 26 and 28 may take the form of a conductive plate whichpreferably exhibits a curved cross-section so that the surfaces thereofare parallel to the peripheral portions of the photoreceptor surfaceadjacent thereto; however, development electrodes in the form of a flatplate may alternatively be used. The first and second developmentelectrodes 26 and 28 would each generally be continuous and hence wouldpreferably provide a continuous surface area disposed opposite toperipheral portions of the photoreceptor 2, although, laminated orapertured surface configurations could also be used. In addition, thesurfaces of the first and second development electrodes 26 and 28 andparticularly the surface of the first development electrode 26 could begrooved in the direction of developer material flow to thereby increasethe flow rate thereof. Each of the first and second developmentelectrodes 26 and 28 would have a width, as measured along the axialwidth of the photoreceptor 2 or into the paper, which is substantiallyconterminous to that of the photoreceptor and would have a typicallength, as measured along the periphery of the photoreceptor, ofapproximately 1 inch when relatively small diameter photoreceptor drumsare employed. The length of each of the development electrodes 26 and 28may widely vary and may generally be considered to admit of a combinedlength equal to approximately 25 percent of circumference of thephotoreceptor drum provided such length may be otherwise accommodated bythe housing employed. For instance, if a conventional 25 inchcircumference photoreceptor drum were employed, the combined lengths ofthe first and second development electrodes 26 and 28 could be about 6inches and naturally larger drums would permit longer electrode lengths.In high speed electrophotographic equipment more development electrodearea is needed to obtain greater development time and hence relativelylarge development electrodes would be preferred. Other factors to beconsidered in the choice of the lengths for the first and seconddevelopment electrodes 26 and 28, as well known to those of ordinaryskill in the art, are the bias levels associated therewith, the size ofthe latent electrostatic image formed, the spacing of the first andsecond development electrodes 26 and 28 from the photoreceptor 2 as wellas the potential levels generally associated with the latentelectrostatic image formed. It should be noted, however, that thelengths of the first and second development electrodes 26 and 28 shouldbe selected so that the combined length thereof will not prohibitadequate spacing therebetween or a free gravitational flow of developermaterial. Furthennore, if the lengths of the first and seconddevelopment electrodes 26 and 28 are selected in a manner so as to beunequal, it is preferred that the length of the second developmentelectrode 28 be larger than that of the first development electrode 26because, as shall be seen below, the second development electrode 28accomplishes low density and extended area development and hence fineimage development which should be attended by greater field modifyingcharacteristics.

The first development electrode 26 would typically be spacedapproximately two-tenths of an inch from the surface of thephotoreceptor 2, although any spacing from eight one hundredths of aninch to one half an inch would be appropriate. Optimum spacing of thefirst development electrode means 26 would substantially vary inaccordance with the bias used therewith, the size of the developermaterial particles and particularly the carrier bead diameters employed,the flow rate of the developer material from the hopper 24 and thepotential levels associated with the latent electrostatic image formed;however, as shall be rendered apparent below, the actual spacing fromthe photoreceptor 2 selected for the first development electrode 26should be determined, upon a consideration of each of the foregoingfactors, with a view toward maintaining a high rate of developermaterial flow even though the development of extended area and lowdensity portions of the latent electrostatic image are substantiallyreduced. The first development electrode 26 is connected to a suitablesource of potential V which may here take the form of a conventionald.'c. source. The source of potential V according to the puredevelopment mode of operation illustrated in FIG. 1 applies a bias tothe first development electrode 26 which may be selected at a levelequal to or slightly greater than the potential level associated withthe background of the latent electrostatic image formed so that suchbackground portions of the latent electrostatic image would appearneutral or of the same charge polarity as the triboeleclit tricallycharged toner material whereby the background would develop as purewhite. Thus, in a typical case, if the background of latentelectrostatic images formed on the photoreceptor 2 generally residedbetween 50 and 100 volts positive, the potential source V would beselected so as to apply 100 to 300 volts to the first developmentelectrode 26.

The second development electrode 28 here acts, as will be seen below, inthe presence of a reduced developer material flow rate to achieve finedevelopment of the extended and/or low density portions of the latentelectrostatic image formed on the photoreceptor 2. Accordingly, thespacing of the second development electrode 28 from the surface of thephotoreceptor 2 is optimized for such fine development and is typicallydisposed at approximately one-half the distance from the photoreceptor 2as the first development electrode 26. Thus, the second developmentelectrode 28 would typically be spaced approximately seventy-fivethousandths of an inch from the surface of the photoreceptor 2 with aspacing range of four hundredths of an inch to a quarter of an inchbeing generally available to compensate for such factors as the flowrate of developer material, the bias level associated with the seconddevelopment electrode, the size of the developer material particles andparticularly the carrier bead diameters employed and the potentiallevels associated with the latent electrostatic image formed. The seconddevelopment electrode 28 is connected to a suitable source of potentialV which may take the same form as potential source V The source ofpotential V acts to apply a potential level to the second developmentelectrode 28 which may also be selected as equal to or greater than thepo tential level associated with the background of the latentelectrostatic image formed so that the development of such latentelectrostatic image which takes place at the second developmentelectrode 28 will develop such background portions as pure white. Thus,the voltage level applied by the potential source V to the seconddevelopment electrode 26 may here be precisely the same as that appliedby the potential source V to the first development electrode 26,although highly advantageous results may sometimes be obtained byslightly raising or lowering the potential applied to the seconddevelopment electrode 28 from that applied to the first developmentelectrode 26 so that more controlled development may be achieved. Aswill be apparent, when the same bias levels are applied to the first andsecond development electrodes 26 and 28, a common potential source maybe substituted for the individual sources illustrated.

Upon completion of the rotation of the periphery of the photoreceptor 2through the development station 8, the portion of the photoreceptor 2having a developed image thereon next proceeds to the image transferstation 110 whereat image transfer takes place. The image transferstation 110 comprises a transfer member 38 and charging means 39 M forimposing a predetermined charge on such transfer member 38 to therebyaffect transfer of the toner image from the surface of the photoreceptor2 to the transfer member 36. The transfer member 38 may take the form ofa sheet, web or drum formed of suitable transfer material such as paper,plastic or any of the other well-known materials conventionally employedas transfer materials. The charging means 39-4lll may take the form of aplurality of half-round corotrons, as illustrated, which may bestructured in the same manner as the charging means illustrated at thecharging station 2 and described above. Alternatively, any otherconventional form of charging means such as those discussed inconjunction with the charging station 2 may be employed in place of thecharging means 39-411 illustrated in H0. ll, it being appreciated thatany charging means capable of imposing a predetermined charge level onthe surface of the transfer member 38 may be employed at the imagetransfer station it The charging means 39-411 illustrated in FIG. l areconnected to a suitable source of potential V which again may take theform of a conventional d.c. supply. Although in the embodiment of theinvention illustrated in FIG. 1, only a single polarity chargingoperation is indicated at the image transfer station 10, it will beappreciated by those of ordinary skill in the art, that image transfermay be accomplished with a subsequent stripping operation by using apair of oppositely poled corotrons connected to a floating supply suchas described in US. Pat. No. 3,244,083, issued to R. W. Gundlach on Apr.15, 1966.

Upon completion of the electrophotographic operation at the transferstation lltl, the image portion of the photoreceptor 2 next passes tothe cleaning station 12 where residual toner particles are removed fromthe surface of the photoreceptor 2 preparatory to the subsequentformation of a new latent electrostatic image on the photoreceptor 2.The cleaning station 12 may take the conventional form of rotating furbrush cleaning means, wiper means or cascade cleaning means which act inthe well-known manner to remove residual toner from the surface of thephotoreceptor 2. As each of these techniques is well known to those ofordinary skill in the art, it is here suffieient to simply state thatthe cleaning station 12 acts to remove residual toner particles from thesurface of the photoreceptor 2 to thereby place the same in conditionfor reuse.

in the operation of the embodiment of this invention illustrated in FIG.1, it will be appreciated that a latent electrostatic image of theobject to be copied will initially be formed on the photoreceptor 2 bythe combined action of the charging station l and the exposure station6. The latent electrostatic image is formed, upon the energization ofthe electrophotographic apparatus depicted in FlG. l, by conventionalelectrophotographic processes well known to those of ordinary skill inthe art. Thus, when the electrophotographic appara tus illustrated inFIG. l is energized, the charging means 18-20 present at the chargingstation 4 will impose a charge on the portions of the photoreceptorpassing therebeneath by the application of ion charging current thereto.The surface of the photoreceptor 2 will thus be charged to a uniformpotential by the action of the charging means 18-20 which areillustrated in FIG. l as comprising conventinal corotrons. Accordingly,it will be appreciated, that the charging station 4i acts in theconventional manner to sensitize the surface of the photoreceptor 2 bycharging the same to a uniform potential which for the purposes of theinstant disclosure may be assumed to be 1,000V. although any suitablemagnitude and/or polarity charge configuration may be selectivelyimposed on the surface of the photoreceptor 2. Upon the completion ofthe charging or sensitizing operation which takes place at the chargingstation 4, the rotation of the photoreceptor 2 in the directionindicated by the arrow A will bring the previously charged portions ofthe photoreceptor 2 into an operative relationship with the exposurestation 6. The exposure station 6 acts in the well-known manner toselectively expose the pontoconductive portion of the photoreceptor 2,which here takes the form of the insulating member 14, toelectromagnetic radiation from the object to be copied. Thus, theexposure station 6 acts to image a light and dark pattern representativeof the image to be formed on the charged surface of the insulatingmember 14 of the photoreceptor 2. Upon exposure, the photoconductiveinsulating member 14 is rendered selectively conductive, in thewell-known manner, so that the charge levels on the light struckportions thereof are substantially reduced while charge levels onportions thereof which correspond to dark portions of the image area aremaintained at the relatively high charge levels imposed at the chargingstation 4. Thus, in this well-known manner, a latent electrostatic imageis formed on the photoreceptor 2 by the combined action of the chargingstation 4 and the exposure station 6 disposed about the peripherythereof. For the purposes of the development operation set forthhereinafter, it may be assumed that the charge level variation of thelatent electrostatic image formed manifests voltage levels ofapproximately 800 volts at portions of the image area corresponding todark portions of the light and dark exposure pattern while the lightstruck portions of the image area on the surface of the photoreceptor 2corresponding to background areas of the image reside at approximately100 volts. The image area or the portions of the photoreceptor 2 havingthe latent electrostatic image formed thereon are next brought into anoperative relationship with the development station 8 due to therotation of the photoreceptor 2 in the direction indicated by the arrowA.

The description of the operation of the development station 8 willassume that a positive to positive development mode is desired and thatthe photoreceptor is positively charged. Therefore, the developermaterial selected is such that negatively charged toner particles arerelied upon; however, as will be apparent to those of ordinary skill inthe art, a negative to positive mode of development could bealternatively elected by the selection of developer materials resultingin positively charged toner particles or that with a negatively chargedphotoreceptor the developer materials selected could be such thatnegative to negative or positive to negative modes of developmentresult. Accordingly, at the developer station 8, the reservoir 30 willbe filled to a predetermined level with developer material whosecharacteristics are such that the carrier beads exhibit a positivecharge while the toner material acquires a negative charge due to thephenomenon of triboelectrification. Furthermore, as the reservoir 30 isfilled to a predetermined level with developer material, the hoppermeans 24 due to the action of'the conveyor means, not shown, will alsobe filled to a desired level with such developer material. As was statedabove, the

ramp 34 present in the hopper means 24 exhibits an.

angle of inclination calculated to insure that developer material 32will be cascaded or flowed over the surface of the photoreceptor 2disposed thereunder at a flow rate which is relatively high when thecore material of the selected carrier material is considered. Thus, asindicated in FIG. 1, developer material in the form of carrier beadshaving oppositely charged negative toner material clinging to thesurface thereof due to the electrostatic force of attractiontherebetween will be cascaded over the surface of the photoreceptor 2 ata flow rate which is preferably high with respect to the core materialselected for the carrier beads. For the purposes of the instantdisclosure, the flow rate at which developer material is supplied fromthe hopper means 24 to the photoreceptor 2 may be assumed to take anexemplary value of 50 grams per inch per second although wide variationstherein are available for the reasons aforesaid.

The developer material delivered to the photoreceptor 2 from the hoppermeans 24 will flow over the surface of the photoreceptor 2 in the formof carrier beads having a plurality of toner particles clinging to thesurface thereof due to the electrostatic force of attraction exhibitedtherebetween. As the developer material 32 is applied to the surface ofthe photoreceptor 2, the latent electrostatic image formed on thesurface of the photoreceptor 2 will be initially subjected to the fieldmodifying effects of the first development electrode 26 which is biasedby the potential source V to a voltage calculated to insure that thebackground portions of the latent electrostatic image on thephotoreceptor 2 will develop out as pure white. Therefore, as thebackground portions of the latent electostatic image have here beenassumed to reside at potential levels ranging from 50 to volts, it mayhere be assumed that the first development electrode is biased to alevel of 200 volts although any value between 50 and 500 volts would beavailable depending on the result desired. The first developmentelectrode 26 is spaced an appropriate distance from the surface of thephotoreceptor 2, which may here be assumed to be two tenths of an inch,so as to maintain a high developer material flow rate even through thespacing appropriate for such high flow rate will reduce the fieldmodifying effects of the first development electrode 26 so that thedevelopment of the low density and extended area portions of the latentelectrostatic image are substantially reduced. This high flow rate maybe further assured by grooving or otherwise structuring the firstdevelopment electrode 26 to increase the flow rate. Therefore, as theseconditions obtain in the space intermediate the periphery of thephotoreceptor 2 and the first development electrode 26, the developermaterial being cascaded over the surface of the photoreceptor 2 in thevicinity of such first development electrode will proceed at a ratewhich is not substantially impeded; however, the developmentaccomplished at the surface of the photoreceptor underlying the firstdevelopment electrode 26 will be such that only the portions of thelatent electrostatic image exhibiting substantial voltage contrasts willmanifest electrostatic forces capable of overcoming the carrier head totoner bond of the developer material so that toner material is depositedthereon. Thus, the first development electrode 26 tends to optimizedeveloper material flow in the space influenced thereby, while the fieldmodifying effects produced for the development of a continuous tonelatent electrostatic image are reduced so that toner material will notbe here deposited on low density or extended areas of the latentelectrostatic image undergoing development.

The second development electrode 28 is closely positioned with respectto the surface of the photoreceptor 2 and may be biased in the puredevelopment embodiment of this invention illustrated in FIG. 1 to thesame level as was the first development electrode 26. Thus,

the second development electrode 28 may here be assumed to be spacedfrom the surface of the photoreceptor 2 by a distance of approximatelyseventy-five thousandths of an inch and to be biased to a level ofapproximately 200 volts so that the flow of developer material will besubstantially reduced while the field effecting characteristics of thesecond developer electrode 28 are substantial. When these conditionsobtain, as is indicated in FIG. 1, the flow rate of developer materialintermediate the second development electrode 23 and the surface of thephotoreceptor 2 will be reduced to approximately grams per inch ofphotoreceptor per second with the conditions assumed above while excessdeveloper material applied to the photoreceptor 2 from the hopper means24 will flow directly into the reservoir 30 by passing through a gapintermediate the first and second development electrodes 26 and 28 inthe manner indicated in FIG. 1. Furthermore, as will be appreciated bythose of ordinary skill in the art, the first development electrode 26may be positioned at a precise peripheral location about the surface ofthe photoreceptor and structured and/or shaped so as to assure that themain portion of the flow of developer material between the firstdevelopment electrode 26 and the photoreceptor 2 is directed through thegap between the first and second development elec trodes 26 and 28 intothe reservoir 30. Thus, under these conditions, the second developmentelectrode 28 will have a very substantial effect on the field exhibitedby the latent electrostatic image on the photoreceptor 2 so that allportions of the electrostatic latent image will be developed out, forthe reasons aforesaid, substantially in proportion to the charge levelsthereon. Accordingly, the increased field exhibited by all portions ofthe latent electrostatic image residing at a voltage level which exceedsthe bias level of the second development electrode 23 when the latentelectrostatic image is under the influence'of the second developmentelectrode 28 will readily overcome the carrier to toner bonds ofcascading developer material in proportion to their magnitude wherebyall portions of the latent electrostatic image except for the backgroundareas thereof will have a dense toner deposition thereon in accordancewith the charge levels associated with such image portions. Furthermore,as the flow rate of the developer material in the region between thesecond development electrode 28 and the photoreceptor 2 is substantiallyreduced, no bunching of developer material will occur so that theaforementioned deleterious effects thereof will be avoided.

Thus it will be seen that the reliance upon the first and seconddevelopment electrodes 26 and 28 taught by the instant invention enablesboth large amounts of developer material to be cascaded over the imagearea of the photoreceptor 2 to assure the presence of large volumes oftoner material and the attendant high development speeds associatedtherewith while low density and extended area portions of the latentelectrostatic image formed are developed out under the influence of thesecond development electrode 28 which exhibits substantial fieldmodifying effects to assure that development is completed underconditions which are optimum for the development of continuous toneimages exhibiting solid area coverage. Furthermore, due to the nature ofthe first and second development electrodes 26 and 28, the optimizeddevelopment specified herein is carried out under conditions wherebyhigh developer material flow is maintained to assure that high development speeds are achieved without impeding this flow in a manner wherebybunching of the developer material will take place and adversely affectthe development of the image. Accordingly, the methods of developmentand the apparatus therefor in accordance with the teachings of thisinvention as illustrated in combination with the exemplaryelectrophotographic apparatus shown in H6. 1 enables advantageouscascade development techniques to be employed in the development ofcontinuous tone latent electrostatic images without compromising theimage formed or the speed at which development is carried out inresponse to the requirements of the development electrode meansutilized.

Upon the completion of the development step carried out at thedevelopment station 8 in the foregoing manner, toner material isdeposited on the image area of the photoreceptor 2 in accordance withthe charge configuration of the latent electrostatic image formed due tothe electrostatic force of attraction between the negatively chargedtoner particles and the positively charged latent electrostatic image.When the image area of the photoreceptor 2 arrives at the transferstation 10, the transfer member 33 will be disposed on the surface ofthe photoreceptor 2 in such manner that the transfer member will be inphysical contact with the toner image formed at only a few points on thesurface thereof due to the absence of a strong force of attrac tionbetween the adjacent surfaces of the transfer member 38 and thephotoreceptor 2 while the remaining adjacent surface portions may beseparated by an air gap of several microns. The transfer member 38, asaforesaid, may for the purposes of this disclosure be considered to takethe form of the web illustrated in FIG. 1 and is adapted for motion inthe direction indicated by the arrow B in such manner that the speed ofthe portion of the transfer member 38 adjacent to the surface of thephotoreceptor 2 is equal to the angular velocity of the photoreceptor 2.When the successive portions of the photoreceptor 2 having the transfermember 38 disposed thereon are displaced so as to be in a chargingrelationship with the charging means 394i, portions of the transfermember 38 in a charging relationship therewith will receive positive ioncharging current along the width thereof so that such surface portionsare positively charged to a uniform potential. The positive chargesapplied to the surface of transfer member 38 will migrate in thewell-known manner to the opposite surface thereof adjacent to the tonerimage and hence will induce negative charges in corresponding portionsof the conductive backing R6 of the photoreceptor 2. The charge on theportions of the transfer member 36 receiving positive ion chargingcurrent from the charging means 39-41 will produce a very substantialforce of attraction between corresponding portions of the adjacentsurfaces of the transfer member 38 and the photoreceptor 2 therebybringing such portions of the transfer member 38 into intimate contactwith the portions of the toner image on the adjacent portions of thephotoreceptor 2. When these conditions obtain, the field strengthbetween the charged portion of the transfer member 36 and the adjacentportion of the photoreceptor 2 will be sufficient to cause most of thenegatively charged toner to be transferred from the photoreceptor 2 tothe transfer member 3%. This operation will be continued for suceessiveportions of the transfer member 38 on a continuous basis until theentire toner image has been transferred to the transfer member 38. Thus,in the well known manner, the toner image formed on the photoreceptor 2by the developing step carried out at the development station 8 istransferred to the transfer member 38 so that only residual tonermaterial from the step of development remains on the surface of thephotoreceptor 2.

The image portion of the photoreceptor 2 is next brought into anoperative relationship with the cleaning station 12 so that residualtoner material may be removed from the image portion of thephotoreceptor 2 prior to reuse. The cleaning station I2 may be entirelyconventional, as aforesaid, so that if nontacky toner material is reliedupon, the image portion of the photoreceptor 2 may be wiped by aconventional rotating fur brush or cotton wiping means may be employed.Alternatively, the cleaning station 12 may employ cascade cleaningtechniques wherein granular material, which acts similarly to thecarrier beads in a two component developer material, is cascaded overthe image portion of the photoreceptor in the well-known manner andresidual toner particles are attracted from the surface of thephotoreceptor 2 by triboelectric attraction without danger of marring orscratching the surface of the photoreceptor. Upon the completion of thecleaning step, the electrophotographic equipment illustrated in FIG. 1may be again utilized to form a toner image of an original to be copiedwhereupon the steps of the forma tion of a latent electrostatic image,development, transfer and cleaning may again be repeated.

In the typical electrophotographic equipment illustrated in FIG. 1 anddescribed above, an individual cleaning stage for the removal ofresidual toner was provided and hence the exemplary embodiment of thisinvention disclosed in association therewith was directed solely to thedevelopment aspects of the present invention wherein a split or firstand second development electrode configuration was relied upon to allowcascade development techniques or the like to be utilized for thedevelopment of a continuous tone image without comprising thedevelopment technique employed to allow for the utilization ofdevelopment electrode means. However, the attributes of the instant invention are not limited to advantageous development techniques but, inaddition thereto, allow cleaning and development to take place within asingle processing stage. An exemplary embodiment of this inventionwherein cleaning and development are combined within a single processingstage is described below in conjunction with FIG. 2.

FIG. 2 illustrates another exemplary embodiment of the present inventionwehrein methods of development cleaning and apparatus therefor are shownin combination with a modified version of the electrophotographicequipment shown in FIG. 1. As will be noted upon an inspection of FIG.2, the structure illustrated therein corresponds essentially to thatshown in FIG. 1 with the single exception that the cleaning station 12has been omitted since, as will be seen below, cleaning is performed inthis embodiment of the present invention within the development station.Therefore, in order to avoid undue repetition and reiteration, structureillustrated in FIG. 2 which corresponds to similar structure alreadydescribed in conjunction with FIG. 1 has retained previously utilizedreference designations and the description of structure in FIG. 2 commonto that illustrated in FIG. 1 and described above shall proceed by wayof reference to the descriptive material set forth in connection withFIG. 1. Accordingly, it will thus be appreciated that any of thealterations, modifications or varying parameter ranges set forth in thedescriptive material presented in association with FIG. I shall be fullyapplicable to the structure depicted in FIG. 2 unless otherwisespecified.

The electrophotographic processing equipmentillustrated in FIG. 2comprises a photoreceptor 2, a charging station 4, an exposure station6, a development station 8 and a transfer member l0.-The photoreceptor2, charging station 4, exposure station 6 and transfer 8K3: tion 10 mayeach take the same structural form, perform the same functions and admitof the same variations as the correspondingly annotated structureillustrated in FIG. 1. Similarly, the development station 8 is disposedabout a peripheral portion of the photoreceptor 2 in the same manner aswas described in conjunction with FIG. 1 and comprises hopper means 24,a reservoir 30 and conveyor means (not shown) all of which maystructurally take the same form and perform the same function as wasdescribed above. In this embodiment of the invention, however, the ramp34, as present within the hopper means 24 is preferably disposed at anangle of inclination which will assure that the flow rate at whichdeveloper material is cascaded over the surface of the photoreceptor 2is high because not only are such high flow rates desirable from thestandpoint of rapid development and the formation of densely populatedtoner images, but in addition thereto, as is well known to those ofordinary skill in the art, the removal of residual toner images bycascading developer material in an electroded system is enhanced byrelatively high flow rates for the developer material utilized. Thus,although flow rates for the developer material 34 in the rangesspecified above in conjunction with FIG. 1 are fully applicable to theinstant embodiment of the present invention, it is preferred that arelatively high rate be selected, again considering the core material ofthe carrier chosen, so that more efficient development cleaning willresult.

In addition, the development station 8 also includes first and seconddevelopment electrodes 46 and 28. The first and second developmentelectrodes 46 and 28 are disposed about the periphery of thephotoreceptor 2 and within the development station 8 in precisely thesame manner as was described above in conjunction with the first andsecond development electrodes 26 and 28 depicted in FIG. 1. Furthermore,the first and second development electrodes 46 and 28 as shown in FIG. 2may also take the same structural configurations and exhibit the samedisplacements from the surface of the photoreceptor 2 as well as thevariations thereof which were described in conjunction with the firstand second development electrodes 26 and 28 shown in FIG. 1; however, aslightly greater displacement than specified above may be hereappropriate for the first development electrode 46 to accommodate alarger flow rate of developer material 32 from the hopper means 24. Thesecond development electrode 28 here serves in the presence of asubstantially reduced flow of developer material, of the same magnitudeand obtained in the same manner as described in conjunction with FIG. 1,to develop extended area and/or low density portions of the latentelectrostatic image formed and hence, as indicated by the referencenumeral associated therewith, takes the same form, performs the samefunctions and admits of the same variations as the second developmentelectrode 28 described above in conjunction with FIG. l. The seconddevelopment electrode 28 shown in FIG. 2 is connected to a source ofpotential V which supplies an appropriate background bias level theretoas aforesaid.

The first development electrode 46, however, here performs the dualfunctions of cleaning the surface of the photoreceptor 2 and thedevelopment of portions of the electrostatic image displaying largecharge variations. The cleaning function of the first developmentelectrode 46 is achieved by a reliance on the ability of cascadingdeveloper material and more particularly carrier beads therein, whichare not associated with a sufficiently large number of toner particlesto render the resultant developer particle electrically neutral, toscavenge residual toner particles from the surface of the photoreceptor2 when such residual toner particles are not strongly adhered to thesurface of the photoreceptor 2 by a subsequently established latentelectrostatic image. To assure the presence of a sufficient number ofcarrier beads associated with less than a full complement of tonerparticles clinging thereto, such cleaning of residual toner particlesfrom the surface of the photoconductor 2 should take place not only inthe presence of a high flow rate but additionally a sufficiently largebias on the surface of the first development electrode 46 to effectivelyencourage the removal of toner particles from the carrier beads in thecascading developer material. For this reason, the first developmentelectrode 46 is connected to a potential source V which may be entirelyconventional, and preferably applies a substantial bias level to thefirst development electrode 46. Thus, while in the FIG. I embodiment ofthis invention the bias level applied to the first development electrode26 was related to the background level of the latent electrostatic imageformed, here, the bias level applied to the first development electrode46 may range from 0 to a 1,000 volts and preferably should be at orabove the charge levels of portions of the latent electrostatic imagecorresponding to the dark portion of the light and dark exposurepattern. The develop ment function of the first development electrode46, wherein only the portions of the latent electrostatic imagedisplaying high voltage contrasts are developed due to the substantialdistance between the surface of the photoreceptor and the firstdevelopment electrode occurs in the same manner as was described inconjunction with FIG. I; however, here the development which takes placeat the first electrode means 46 may be slightly less efficient than wasthe case in the FIG. 1 embodiment because conditions are preferablyoptimized in favor of cleaning with the high flow rate and high biaslevel rather than in terms of development. Thus, in the embodimentillustrated in FIG. 2, the development electrode configuration accordingto the instant invention functions to achieve both cleaning and reduceddevelopment at the first development electrode 46 while fine developmentof low density and extended area portions of the latent electrostaticimage is carried out at the second development electrode 28 in the samemanner as accomplished in the FIG. I embodiment.

In the explanation of the operation of the electrophotographicprocessing equipment illustrated in FIG. 2 and more particularly inthe-description of the development and cleaning mode of operationillustrated therein, it will be again assumed that a positive topositive development mode is desired; it being appreciated that any ofthe alternatives thereto mentioned with re gard to FIG. I will bereadily available herein. Thus, the developer material illustrated aspresent within the reservoir 30 and the hopper means 24 will be assumedto be such that negative toner partices are available for the formationof a toner image. Furthermore, it will be additionally assumed thatprior to the cycle of operation about to be considered, a previous cycleof operation was performed wherein a toner image was developed andtransferred so that residual toner particles are present on the surfaceof the photoreceptor 2 which is to undergo the cycle of operation setforth below.

In the operation of the embodiment of this invention illustrated in FIG.2, it will be appreciated that a latent electrostatic image of theobject to be copied is formed on the surface of the photoreceptor 2 bythe steps of charging the photoreceptor 2 to a uniform potential andthereafter selectively discharging such photoreceptor 2 in accordancewith a light and dark exposure pattern representing the object to becopied. This may be accomplished in the conventional manner by thecharging station 4 and the exposure station 6 by the same techniquesdescribed in connection with FIG. I. However, prior to establishing thelatent electrostatic image, it may be advantageous to flood the surfaceof the photoreceptor 2 with electromagnetic radiation, in the well-knownmanner, so that the photoconductive layer is fully discharged prior tothe formation of a new latent electrostatic image. In any event, alatent electrostatic image is formed in the same manner as described inconjunction with FIG. I and the image portion of the photoreceptor 2 isthen displaced in the direction of ro tation indicated by the arrow A sothat development and cleaning may be carried out at the developmentstation 8. It should be noted that under the conditions which hereobtain, the image portion of the photoreceptor 2 not only has a latentelectrostatic image formed thereon but, in addition thereto, residualtoner particles may be present on the surface of the photoreceptor 2 dueto previous use.

When the image portion of the photoreceptor 2 arrives at the developmentstation 6, the reservoir 30 and the hopper means 24 will both be filledto their respective predetermined levels with developer material of thetype described in conjunction with FIG. I. Accordingly, developermaterial will be cascaded over the surface of the photoreceptor 2 by thehopper means 24 at a rate which may be again assumed to be 50 grams perinch per second although, as aforesaid, other high rates of flow for thedeveloper material are clearly available depending on the density of thecarrier beads relied upon therein. As the developer material is cascadedover the surface of the photoreceptor 2 and follows a path intermediatethe first development electrode 46 and the photoreceptor 2, carrierbeads not having a full complement of toner particles associatedtherewith will scavenge residual toner particles from the portions ofthe surface of the photoreceptor which do not correspond to portions ofthe newly formed latent electrostatic image having large charge contrastwhile toner clinging to the surface of other carrier beads will depositon surface portions of the photoreceptor 2 corresponding portions of thenewly formed latent electrostatic image which do exhibit large chargecontrast.

The scavenging thus carried out accomplishes the removal of residualtoner particles while the toner deposition not only achieves developmentof portions of the latent electrostatic image exhibiting large voltagecontrasts but in addition thereto frees more carrier beads for cleaningpurposes. As was stated above, it is preferred in the instant embodimentof this invention that the bias applied to the first developmentelectrode 46 by the potential source V be high so as to aid in theremoval of residual toner emaining on the surface of the photoreceptorfrom the development and subsequent transfer of a previously formedlatent electrostatic image. This high bias level on the firstdevelopment electrode 46 not only acts to modify the field associatedwith the latent electrostatic image for the purposes of development in asimilar manner to that described for the first development electrode 26in conjunction with FIG. 1, but in addition thereto, aids in thecleaning function carried out by the first development electrode 46 bymaking available more carrier beads for cleaning purposes by aiding inthe removal of toner therefrom. The manner in which the firstdevelopment electrode 46 aids cleaning will be understood by anappreciation that so long as a positive bias level is imposed on thefirst development electrode 46 some toner particles will be removed fromcascading carrier beads and drawn thereto whereby additional carrierbeads not having a full complement of toner particles associatedtherewith will be freed for the purposes of development and as the biaslevel on the first development electrode 46 is increased the number ofcarrier beads thus rendered available will be substantially increased.When the bias level associated with the first development electrode 46is at or only a few hundred volts above the background level of thelatent electrostatic image and high flow rates as indicated above areutilized, soft, gentle cleaning will take place while, in similar mannerto the embodiment of the invention disclosed in FIG. 1, suitable coarsedevelopment of portions of the latent electrostatic image exhibitingappropriate voltage contrasts will take place. However, as the biaslevel associated with the first development electrode 46 is increasedsubstantially above the background level of the latent electrostaticimage formed, the cleaning rate will be markedly increased due to themanner in which the first development electrode 46 will overcome thetoner-carrier bead bond and draw toner particles thereto, but underthese conditions less development will take place in the vicinity of thefirst development electrode 46 as more portions of the latentelectrostatic image will appear negative with respect to the firstdevelop: ment electrode 46. Furthermore, in this regard, the conditionsappropriate for cleaning and development which take place under theinfluence of the first development electrode 46 can be so weighted infavor of cleaning, for instance by placing a bias level on the firstdevelopment electrode 46 which exceeds the level of all portions of thelatent electrostatic image, that the rate of cleaning will besubstantially increased but with a corresponding reduction in the rateof development. However, should it be desired to increase the rate ofcleaning to the point where the second development electrode 28 can nolonger compensate for the development lost, a third developmentelectrode spaced for high rates of developer material flow could beinterposed between the first and second development electrodes 46 and 28and the first development electrode 46 may be coated with a layer ofconductive teflon or the like to avoid toner buildup thereon. Thus, itis seen that cleaning of residual toner material from the photoreceptoras well as development of portions of the latent electrostatic imageexhibiting substantial voltage contrasts is achieved at a high flow rateof developer material under the influence of the first developmentelectrode 46.

As the second development electrode 28 is displaced, as aforesaid, atapproximately one-half the distance from the surface of thephotoreceptor 2 with respect to the first development electrode 46, theflow rate of developer material cascading between the second developmentelectrode 28 and the surface of the photoreceptor 2 will be markedlyreduced. Such reduced flow rate may be assumed to be the same asdescribed in connection with the FIG. 1 embodiment; however, as will beappreciated by those of ordinary skill in the art, the flow rate andhence the spacing of the second development electrode 28 as isappropriate therefor may be increased to compensate for extremelyreduced coarse development at the first development electrode 46. Thedeveloper material representing the difference in flow rates between thefirst and second development electrodes 46 and 28 will cascade intoreservoir 30 through the path indicated in FIG. 2 between the first andsecond development electrodes 46 and 28 while developer material notutilized in the fine development which takes place at the seconddevelopment electrode 28 flows into the reservoir 30 through the flowpath indicated. Thus, both cleaning and development are hereaccomplished within the development station 8 by the first and seconddevelopment electrodes 46 and 28 and the proportioned flow rates ofdeveloper material enabled thereby. Upon the completion of thedevelopment and cleaning operation carried out at the developmentstation 8, the toner image formed may be transferred and subsequentlyfixed at the transfer station 10 in the same manner as was described inconjunction with FIG. I. Thereafter, a new latent electrostatic imagemay be formed as no independent cleaning operation is required accordingto this embodiment of the present invention.

Although the methods of development and development cleaning as well asapparatus therefor taught by the present invention have been disclosedin conjunction with two detailed exemplary embodiments thereof, it'willbe appreciated that many modifications and alterations to the techniquesset forth are available and hence contemplated by the instant invention.For instance, although the development electrode configurationsdescribed herein have been depicted as providing a continuous surfacewhich is parallel to the surface area of the photoreceptor; grooved,laminated, apertured or other discontinuous surface configurations forsuch electrode structure may be employed and this is particularly sowhen such surface configurations are relied upon to enhance the flowrate of developer material or the conveyance thereof to the reservoir orsump. Furthermore, the development electrode structure need not exhibita surface area which is parallel to the surface of the photoreceptor normust the development electrode structure exhibit a surface areadisplaying a constant displacement from the surface of thephotoreceptor. In addition, although first and second developmentelectrodes have been illustrated herein, it will be appreciated that asplit, unitary structure may be employed, especially when a common biaslevel is relied upon or alternatively more than two developmentelectrodes may be utilized when it is desired to further increasegraduations with respect to the developer material flow rates, thecoarse and fine development employed and/or the rates associated withcleaning and development. Furthermore, it will be apparent that althougha specific electrophotographic process was set forth herein to providean appropriate environment for the disclosure of the instant invention,the inventive concepts set forth may be relied upon in conjunction withany electrophotographic process which results in the development of atoner image.

While the invention has been described in connection with severalexemplary embodiments, it will be understood that many modificationsthereofwill be read ily apparent to those of ordinary skill in the artand that this application is intended to cover any adaptations orvariations thereof. Therefore, it is manifestly intended that thisinvention be only limited by the claims and the equivalents thereof.

What is claimed is:

l. A method of development comprising the steps of:

cascading developer material at a high flow rate over a surfaceassociated with a latent electrostatic image;

positioning a first development electrode adjacent to said surface at alocation proximate to the application of said developer materialthereto;

spacing said first development electrode a sufficient distance from saidsurface to allow said developer material to pass between said surfaceand said first development electrode at said high flow rate;

positioning a second development electrode adjacent to said surface inthe vicinity of said first development electrode; spacing said seconddevelopment electrode a smaller distance from said surface than saidfirst development electrode to thereby enable only reduced quantities ofdeveloper material to pass between said surface and second developmentelectrode and further spacing said second development electrode asufficient distance from said first development electrode means to allowdeveloper material passing between said first development electrode andsaid surface and exceeding said reduced quantity to pass therebetween:and

applying a reference potential to said first and second developmentelectrodes to enable said first and second development electrodes tomodify fields exhibited by said latent electrostatic image associatedwith said surface.

2. A method for developing a latent electrostatic image on aphotoreceptive surface comprising:

cascading a developer stream over the image between a first electrodeand the photoreceptive surface at a given flow rate,

passing a portion of the developer stream between a second electrode andthe photoreceptive surface at a reduced flow rate while passing theremaining portion of the developer stream between the two electrodes toa developer sump.

3. A method as recited in claim 2 wherein a potential is applied to thefirst electrode which ranges from the potential associated with thebackground of the latent image to above the potential of the latentimage.

4. A method as recited in claim 2 wherein a different potential isapplied to the first and second electrode so that the developer streamessentially acts to clean the photoreceptive surface of residual tonerwhen passing between the first electrode and the photoreceptive surfaceand that portion of the developer stream passing between the secondelectrode and the photoreceptive surface acts to develop the latentimage.

2. @. A method for developing a latent electrostatic image on aphotoreceptive surface comprising: cascading a developer stream over theimage between a first electrode and the photoreceptive surface at agiven flow rate, passing a portion of the developer stream between asecond electrode and the photoreceptive surface at a reduced flow ratewhile passing the remaining portion of the developer stream between thetwo electrodes to a developer sump.
 3. A method as recited in claim 2wherein a potential is applied to the first electrode which ranges fromthe potential associated with the background of the latent image toabove the potential of the latent image.
 4. A method as recited in claim2 wherein a different potential is applied to the first and secondelectrode so that the developer stream essentially acts to clean thephotoreceptive surface of residual toner when passing between the firstelectrode and the photoreceptive surface and that portion of thedeveloper stream passing between the second electrode and thephotoreceptive surface acts to develop the latent image.